Experiments conducted in this laboratory have shown that Laser Doppler Velocimetry (LDV) of red blood cells (RBC) in human retinal vessels is feasible. This technique is non-invasive. LDV gives absolute velocity data when applied to small particles in a transparent fluid. However, the RBC's are large particles and thus the laser light scattering properties are complex. A study of the effects of multiple scattering by the RBC's and of the absorption of the laser light by the hemoglobin upon our measured Doppler Shift frequency spectra from RBC's will be performed in order to enable us to deduce absolute RBC velocities directly from our measurements. A laser velocimeter suitable for routine use in humans will be developed. Data on the flow velocity of RBC's in humans will be collected and determination of the blood flow rate will be attempted.